The rapid gains in digital technology and telecommunications have increased the desirability of having a network in the home to interconnect a multitude of products in the home with each other and to the outside world. The range of available outside services includes interactive services, cable video and audio services, satellite networks, telephone company services, video on demand, and other types of information services. However, penetration of the personal computer into homes in the United States is approximately 33% and only growing slowly, although governments desire more extensive penetration to encourage “telecommuting” and reduce road traffic and pollution. Further penetration of computers in the home will originate from the purchase of consumer entertainment and informational products containing an embedded computer and operating system hidden by an opaque user interface. Such a product is a conventional set-top box.
Set-top boxes are multi-media computers that augment the use of televisions. A conventional set-top box has an external network interface module that connects the set-top box to the external network and data provider. The network interface module has to perform a number of sophisticated functions, such as interfacing to a specific external network, tuning, demodulation, error correcting, video descrambling, recovery of MPEG clock, and encryption and decryption specific to the external network. Consequently, the network interface module is a relatively expensive component of set-top boxes. This expense would be necessary even when a single television is present in the house. However, most homes contain multiple televisions, and providing each with its own set-top box and associated network interface module is a duplication of expensive components.
One of the functions of network interface units is Motion Pictures Experts Group (MPEG) clock recovery, MPEG-1 and MPEG-2 being different accepted standards for transmitting digitized video data. MPEG data is a continuous stream of data normally clocked at 27 MHz. In conventional arrangements, where the network interface unit is coupled by a bus to the set-top electronics, there is little or no danger of degradation of the video signal due to jitter, since a local bus will not introduce substantial jitter. In contrast, if the video data is placed onto a shared network and distributed through a hub, jitter is likely to be introduced by the home network since the video data may be buffered behind other data at certain points in the network. The higher the jitter, the more difficult it is to recover the clock at the set-top electronics as required.
In order to provide a relatively inexpensive home multimedia network, in which MPEG data is available to multiple set-top electronics coupled to the network, it is necessary to accurately recover the MPEG clock at the separate set-top electronics units.